1. Field of the Invention
The present invention relates to a position encoder and, more particularly, to an encoder for obtaining position data of a rotating member or linearly moving member.
2. Description of the Prior Art
Conventionally, an encoder using a movable encoding plate and a fixed sensor is adopted in order to obtain rotational angle data of a rotating shaft or position data of a linearly moving table. One conventional absolute rotary encoder uses an encoding plate having a plurality of concentric tracks on which codes corresponding to position data are colored or coated in light and dark portions. The encoding plate comprises a reflection type or transmission type disk, and a fixed photosensor is provided opposite to the disk. Another known rotary encoder adopts a magnetic sensor and a magnetic disk having a plurality of concentric tracks which are magnetized into north and south poles to correspond to bits of position data.
The conventional encoders have limited precision depending on data density to be recorded on the encoding plate (i.e., the number of bits included in a unit length of a track) and resolution of the sensor or the S/N ratio of a read signal. Since the size of the encoding plate is limited, the resolution is also limited. For example, in a rotary encoder using a light-transmission type encoding disk and a photosensor, an incandescent lamp or a light-emitting diode is used as a light source. A spot size of the read light beam cannot be decreased. In addition, resolution is limited by the size of the light receiving element and extraneous light interference. Similarly, the rotary encoder of the second type using a magnetic disk and a magnetic sensor has limited resolution due to recording wavelength and resolution of a magnetic sensor such as a variable reluctance element.